Encoders have been used to indicate the position or angle of different movable elements, such as the position of a hydraulic piston or the angle of an aircraft wing flap. Electrical encoders have been used, which vary the potential of an electrical signal in a manner that relates to the position or angle of such a movable element. Optical encoders have been used, which modulate the intensity of a light beam in a manner that relates to the position or angle of the movable element.
Electrical encoders are currently used to indicate the position or angle of different control surfaces of many types of aircraft. These electrical encoders, however, are subject to the effects of electromagnetic interference. Electromagnetic interference can cause an electrical encoder to inaccurately indicate the position of a control surface which would affect the flying capability of the aircraft.
Optical encoders are preferable to electrical encoders, because optical encoders are less sensitive to the effects of electromagnetic radiation than electrical encoders. Optical encoders have optical fibers which are capable of carrying much more information than electrical conductors of electrical encoders. Also, optical encoders are lighter in weight and more reliable than electrical encoders. The accuracy of optical encoders, however, has been limited by the stability of the intensity of a light beam which reads the optical encoder.
The inventors have found that optical encoders can be provided with a reference beam and a signal beam using wavelength division multiplexing. These two beams having different wavelengths are transmitted through a common fiber. However, two light sources are required and intensities and wavelengths of these two sources will usually differ with temperature. Passband filters have been used by the inventors to separate the two beams, but transmission coefficients of these filters are not constant with wavelength. Accuracy of the encoder is degraded when source peak wavelengths vary with temperature. Such degradation occurs when light emitting diodes are used, for instance.
Optical encoders have digital or analog encoding tracks. Digital encoding tracks comprise a number of discrete binary tracks arranged in a binary coded decimal or Gray code configuration. A plurality of light sources scan separate beams onto each binary track to read the digital encoding track. The plurality of sources increases the complexity of a digital optical encoder.
Analog encoding tracks comprise a single track having a transmissivity that varies linearly or sinusoidally along the track, for instance. A single light beam passes through the single track to read the analog encoding track.
Electrical encoders and optical encoders typically comprise linear or rotary encoders. A linear encoder comprises a slide on a barrel of a hydraulic piston, for instance. The encoder slide moves longitudinally as the hydraulic piston barrel extends. A detector senses the position of the slide and produces a signal indicating the position of the hydraulic piston. A rotary encoder comprises a disk which connects directly to a wing flap, for instance. The encoder disk rotates as the angle of the wing flap changes. A detector senses the position of the encoder disk and produces a signal indicating the position of the wing flap.
Thus, a need exists for an optical encoder, which is not only insensitive to the effects of electromagnetic interference, but is able to compensate for the instability of intensity of a light beam which reads the optical encoder without using wave division multiplexing.